Single-wall carbon nanotubes are a novel form of carbon. They are closed-caged, cylindrical molecules, approximately 0.5 to 3 nanometers in diameter and a few hundred nanometers long. They are known for their excellent electrical and thermal conductivity and high tensile strength. Since their discovery in 1993, there has been substantial research to describe their properties and develop applications using them.
All methods for single-wall carbon nanotube production involve one or a combination of transition metal catalysts and a carbon-containing feedstock. Some of the methods to make single-wall carbon nanotubes include electric arc, laser ablation of graphite, and gas phase techniques with supported and unsupported metal catalyst.
The method to prepare carbon nanotubes on supported metal catalyst is known as “chemical vapor deposition” or “CVD”. In this method, gaseous carbon-containing feedstock molecules react on nanometer-scale particles of catalytic metal supported on a substrate to form carbon nanotubes. This procedure has been used to produce multiwall carbon nanotubes, however, under certain reaction conditions, it can produce excellent single-wall carbon nanotubes. Synthesis of single-wall carbon nanotubes using CVD methodology has been described in Dai, et al. (1996), Chem. Phys. Lett., 260, p. 471-475, and “Catalytic Growth of Single-Wall Carbon Nanotubes from Metal Particles,” International Pat. Publ. WO 00/17102 A1, published Mar. 30, 2000, each incorporated herein by reference. The single-wall carbon nanotube material that results from a CVD process comprises single-wall carbon nanotubes, residual catalyst metal particles, catalyst support material, and other extraneous carbon forms, which can be amorphous carbon, non-tubular fullerenes, and, in some cases, multiwall carbon nanotubes. The term “extraneous carbon” will be used herein as any carbon that is not in the form of single-wall carbon nanotubes, and can include graphene sheets, non-tubular fullerenes, multiwall carbon nanotubes, partial nanotube forms, amorphous carbon and other disordered carbon.
In many end-use applications for single-wall carbon nanotubes, it is desirable to use high-purity single-wall carbon nanotubes, containing only minimal amounts of residual catalyst metal, extraneous carbon and catalyst support material. Most CVD methods for producing single-wall carbon nanotubes suffer from relatively low product yields and poor economics. In order to produce a high purity single-wall carbon nanotube product, the product must either be purified after synthesis, which usually leads to loss of single-wall carbon nanotubes, or a method must be found that produces an enhanced yield of single-wall carbon nanotubes with a high productivity catalyst. A need remains for a high yield, economically-effective method for producing single-wall carbon nanotubes.
In some end-use applications for single-wall carbon nanotubes, it is desirable to use nanotubes having a particular length distribution. For instance, when blending single-wall carbon nanotubes with liquids, the length distribution of the nanotubes affects the viscosity characteristics of the liquid/nanotube blend. In some end-use applications, a particular distribution of diameters is desired. A method for producing single-wall carbon nanotubes with a particular distribution of lengths and diameters of nanotubes is needed.